1. Field of the Invention
This invention relates to electro-optical light valves (EOLV), and more particularly to light valves which spatially modulate a readout optical beam corresponding to the modulation of a scanning electron beam.
2. Description of the Related Arts
EOLVs have many important applications. One of them is in large screen displays. Based on the electro-optical (EO) effect of an EO layer, such as that made from a solid state EO crystal or from a liquid crystal (LC), a polarized incident light may change its polarization after passing through the EO layer as a result of the application of an appropriate electric field across the EO layer. An EOLV commonly modulates the intensity or polarization of a readout optical beam by writing a corresponding voltage pattern across the EO layer. Several means of voltage pattern writing have emerged. Among them, EOLVs with passive matrix addressing are small in size, but have long response time, low resolution and low optical output; EOLVs with active matrix addressing are a big improvement over those with passive matrix addressing in many aspects, but they still suffer from low optical output, and the related thin film transistor (TFT) matrix is extremely difficult to manufacture at the necessary total resolution; EOLVs with optical scanning on a photo-electric layer through a scanning laser beam or through an optically coupled CRT may have shorter response time and higher resolution than those with matrix addressing, however, they still need additional means to generate the primary light pattern and a multilayer construction to isolate the readout light from the writing light; EOLVs with electron beam addressing have the advantage over all of the previous ones in having high response speed and high resolution without the need for a primary light pattern and for a light isolating multilayer construction.
Earlier type electron beam addressed EOLVs used the secondary electron emission effect under electron beam bombardment to write the voltage pattern. This type of devices is described, for example, in Puan A. Haven, "Electron-Beam Addressed Liquid Crystal Light Valve", IEEE Transaction on Electron Devices, Vol. ED-30, page 489-492 (1983), and in Thomas S. Buzak, et al., "Method of Addressing Display Regions in an Electron-Beam Addressed Liquid Crystal Light Valve", U.S. Pat. No. 4,884,874. This type of devices needs a flood gun or an erasing gun to erase the voltage pattern before the next scanning cycle. This requirement severely limits its achievable resolution. Jan Grinberg, et al., in U.S. Pat. No. 4,728,174, used a partially conductive layer to receive charges from an electron beam to generate the voltage pattern across an LC layer, and the charges are discharged through a conductive grid deposited on the partially conductive layer. As the most active charging and discharging paths are on the input surface of the partially conductive layer, and as this surface is in the proximity of the conductive grid, the conductive grid tends to pin the voltage across the LC layer to an average value. To reduce the voltage pinning effect, the grid openings need to be as large as possible, but this may have the adverse effect of blurring the voltage pattern as the deposited charges spread laterally on their way of discharging to the conductive grid. This apparently obstructs the achievement of high resolution. In their efforts to improve the system, Jan Grinberg, et al., in U.S. Pat. No. 4,826,293, replaced the conductive grid with a thin conductive sheet. However, even with a substantially high voltage electron beam, the distance to which elections impinge into the partially conductive layer after penetrating through the conductive sheet is only about 1-2 .mu.m. The impinged charge layer is so close to the conductive sheet that an image charge pattern with opposite charges is induced on the inner surface of the conductive sheet, which may severely reduce the intensity of the electric field in the LC layer generated by the impinged charges, leading to the blurring of the voltage image across the LC layer. It is also noted that an electron beam with too high voltage may cause excess heat in the nearby LC layer, accelerating its degradation. It is now obvious that, in order to improve the sharpness of the voltage pattern across the LC layer, the deposited or impinged charges need to be substantially closer to the LC layer and substantially farther away from the conductive input layer.